1. Field of the Invention
The present invention relates to isocyanate-free coating compositions containing (1) at least one polymer and/or oligomer selected from the group consisting of acrylic, urethane and urea polymers and oligomers and combinations thereof having a number average molecular weight of at least about 100 and bearing at least two specific aziridine groups and (2) at least one polymer and/or oligomer selected from the group consisting of acrylic, urethane and urea polymers and oligomers and combinations thereof having a number average molecular weight of at least about 100 and bearing at least two specific epoxy groups capable of forming a durable coating under ambient conditions.
2. Brief Description of Prior Art
Known compositions which cure at low temperatures, for example, at ambient conditions, for use as highly durable automotive topcoats are usually based on two-package systems. These include, for example, hydroxy functional components cured with either isocyanate or anhydride functional components.
The use of isocyanate functional components requires that specific precautions be taken based on toxicity considerations. Such necessary precautions can be relatively burdensome in areas where said materials are used but controlled conditions do not exist or are difficult to obtain. One such example of a generally non-controlled environment is an automobile body repair shop. The application of isocyanate-containing materials is relatively hazardous to exposed workers without proper protection. However, isocyanate/polyol cured compositions deliver the quality and durability demanded by the user. If the crosslinking composition is an anhydride/polyol system, the general overall appearance of the cured coating, i.e. gloss and D.O.I. (depth of image), is inferior to isocyanate-cured coatings. Accordingly, there is an urgent need for high quality, durable coating compositions which cure under ambient conditions, are relatively non-toxic and have the aesthetic appearance of an isocyanate-cured composition.
The present invention addresses these issues. The coating compositions disclosed and claimed herein are not isocyanate curing, are relatively non-toxic compared to isocyanate, are curable at ambient temperature and are equal, or even better, in performance and appearance to both isocyanate/polyol or anhydride/polyol cured compositions.
In the prior art, there is disclosed the curing of diepoxides with a combination of diaziridines and diphenols. The epoxies used are aromatic based or glycidyl functional bisphenol A materials. Such epoxies are not suitable for application to a substrate where high durability is required, such as in refinishing of automobile surfaces.
British Patent No. 1,190,979 to Strother discloses a coating composition comprising a diepoxide, a diphenol and a diaziridine. The diaziridine compounds used are those in which the aziridinyl groups are tertiary amine groups, preferably defined by the following formula ##STR4## wherein R is a divalent hydrocarbon radical having 1 to 10 carbon atoms. Diepoxides that are used include diglycidyl ether, the diglycidyl ethers of bisphenols, diglycidyl ether adduct of adipic acid, vinyl cyclohexene dioxide and the diglycidyl ethers of polyhydric aliphatic alcohols. Diphenols, a necessary component of the system, include bisphenol A, bisphenol F, hydroquinone and resorcinol. The present invention does not require diphenols to obtain curing. Curing of the above mixture requires elevated temperatures in the range of 100.degree. to 200.degree. C. for a period of 10 minutes to one hour. The epoxides and diphenols used by Strother contribute to poor durability and therefore are not useful for automotive topcoat compositions. In addition, curing at elevated temperatures further render such compositions unattractive for use in the automotive refinishing industry.
In U.S. Pat. No. 3,763,100, Martin et al relate to coating compositions containing an epoxy-bisphenol adduct, an adduct of an aziridine compound with a polyepoxide and, optionally, a mono-functional secondary amine that can be air dried or baked at temperatures of from about 25.degree. to about 400.degree. C. for about five minutes to about 24 hours. Martin et al thus use an aromatic diphenol and, as with Strother, produce a composition not useful for durable automotive refinish topcoats. In fact, Martin et al's coating is used as a metal primer coating to improve adhesion between said primer and a subsequent topcoat.
Strother et al in U.S. Pat. No. 3,346,533 disclose the use of a diphenol for curing a diaziridine compound with cycloaliphatic 1,2-diepoxides. This patent is similar to British Patent No. 1,190,979 to Strother, supra, and for the same reason neither discloses nor teaches the compositions and processes claimed herein.